See a list of possible causes in order from the most common to the least.

Rheumatoid arthritis (RA) is an autoimmune disease, a disease in which the immune system mistakes the body's own cells for invaders. In RA, the immune system attacks the synovia, the membranes lining the joints.

Gout is a type of arthritis caused by too much uric acid in the blood. When the concentration of uric acid gets too high, sharp urate crystals form and collect in the joints, causing swelling and intense pain.

Tendons, the fibrous tissues that connect muscles to bones, are covered by a protective sheath called synovium, which keeps tendons lubricated. Injury to this area can disrupt this function, causing inflammation.

Bursitis is an inflammation of the bursae, the fluid-filled sacs that help reduce friction where tendons, skin, and muscle tissues meet bones. Inflammation can cause discomfort and limit range of motion.

Juvenile rheumatoid arthritis is a form of chronic arthritis that affects children. It is a long-term autoimmune condition characterized by stiffness and swelling in the joints. Most cases of JRA are mild.

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